If a substance mixture consisting of at least one low-boiling component (A), a medium-boiling component (B) and a high-boiling component (C) is to be separated by distillation the use of simple distillation columns without a dividing wall general necessitates two distillation columns; cf. Chemical Engineering and Processing, 49 (2010), 139-146. In this way in the first distillation column K1 (cf. FIG. 1) the low boiler (A) may be removed overhead. In the column bottom a bottoms product free from low boilers is achieved which may then be further fractionated in a second column K2. The medium boiling component (B) is obtained at the column top and the high boiling component (C) at the column bottom. According to the composition of the mixture to be separated and the separability of the individual components (i.e. their differences in boiling point) the high boiler (C) may also initially be removed at the column bottom of the first column K1 (cf. FIG. 2). The mixture of low boilers and middle boilers (A, B) obtained at the column top is then fractionated in the second column K2. The low-boiling component (A) is obtained at the column top and the middle boiling component (B) at the column bottom.
In many cases the use of a distillation column having at least one dividing wall (hereinbelow dividing wall column) is in terms of capital and energy costs the more economic alternative to the classical separation sequence. With a dividing wall column all components may be separated in one step (cf. FIG. 3). This is made possible by a dividing wall arranged in the middle portion of the column. The mixture to be separated, also referred to as feed, must be added on the opposite side to the product withdrawal for the middle-boiling component. FIG. 4 shows a typical arrangement of the mass transfer elements, structured packings very often being employed. Arranged above the dividing wall (2) is a common mass transfer element (also referred to as a packing bed) (1). It serves for enrichment of the low-boiling component (A) by removal of the medium-boiling component (B). Arranged left and right of the dividing wall are the mass transfer elements (packing beds) 4, 41, 3 and 31. Bed 4 and 31 serve for removal of the low-boiling component (A) so that this component can reach neither the sidestream withdrawal for the middle boiling component (B) nor the region below the dividing wall. Bed 3 and 41 serve for removal of the high-boiling component (C) with the aim of being ideally free of component (C) in the region above the dividing wall and at the sidestream withdrawal for the component (B). The common bed 10 serves for concentration of the high-boiling component (C).
As intimated in Chemical Engineering, August 2014, 40-48 the importance of dividing wall columns has steadily increased since about 1985 and today encompasses not only asymmetrically arranged dividing walls but also applications with more than one dividing wall. For further energy integration intermediate evaporators and/or condensers are possible, as described in WO 2010/039972 A2. This application further describes cf. FIG. 1—the arrangement of two internals (124, 122—“trays”) in the region above the dividing wall. Located between these two internals is a product outlet (128). Both internals are located in a region of the dividing wall column in which ascending vapor and downward-trickling liquid move in countercurrent; thus mass transfer elements and not apparatuses for homogenizing pure vapor streams are concerned.
EP 1 980 303 A2 is concerned with a special reflux divider (2) for dividing wall columns (1). The reflux divider (2) is arranged outside the column (1) (cf. FIG. 1). The document discloses two embodiments of the reflux divider (2); cf. FIG. 2a and FIG. 2b. In both embodiments the column (1) comprises above the dividing wall a liquid collector (13) which comprises a tray (15) and chimneys (16). The document does not disclose that in the region between the end of the dividing wall and the packing (12) a particular homogenization of the ascending vapor takes place. Ascending vapor cannot become commixed in chimneys since the purpose of these is to minimize pressure drop by provision of a high free cross section.
US 2003/0047438 A1 is concerned with special dividing wall columns. FIGS. 1 and 21 show via apparatus (54) and laminas (71) a liquid collector. The mode of action of this liquid collector is described in paragraph [0052]. It is said therein that the ascending vapors are deflected from the column center but nevertheless a sufficient contact of liquid and vapor is ensured in the adjacent mass transfer elements. An apparatus for homogenization of ascending vapor stream is between the upper edge of the dividing wall and a mass transfer element is not disclosed in this document. Laminar liquid collectors as described in this document (cf. 54 and 71 in FIG. 3) are not suitable for homogenization of vapor stream in columns.
EP 2 829 308 A2 is concerned with a liquid distributor (100) for dividing wall columns. FIG. 1 shows the devices in such a liquid distributor inside a dividing wall column directly above the dividing wall (190). Shown as devices in the liquid distributor are inter alia a liquid collector (130) implemented as a chimney tray (134, 136). Due to the multiplicity of chimneys the described liquid collector is unsuitable for commixing of ascending vapor streams.
The manifold possibilities make dividing wall column technology an important component for reducing energy consumption and manufacturing costs in production.
A typical application of dividing wall column technology is purification of isocyanates, in particular tolylene diisocyanate (hereinbelow TDI). Here, in the chemical reaction of the relevant amine (in particular tolylene diamine, hereinbelow TDA) with phosgene using an inert solvent either as a diluent during the reaction and/or as a quenching medium for rapid reaction termination, a crude product is obtained which must then be worked up by distillation. In the case of TDI (cf. EP 1 371 635 B1 and EP 1 413 571 B1) the dividing wall column is generally utilized for separating the TDI from low-boiling components, from the solvent and high-boiling components in order to obtain it as a saleable product. Thus EP 1 371 635 B1 describes a dividing wall column with less than 2% of the low-boiling component phosgene in the feed. The product stream is withdrawn from EP 1 371 635 B1 in the region of the dividing wall, on the side facing away from the feed, with a purity of at least 99.5 wt %. Said stream contains less than 200 ppmw of solvent and/or chlorinated aromatic hydrocarbons, less than 100 ppmw of hydrolyzable chlorine (HC) and less than 40 ppmw of acidic fractions. Generated at the column bottom is a material stream enriched in high-boiling components. EP 1 413 571 B1 describes a process for purifying TDI in which the feed mixture to the dividing wall column contains less than 20 wt % of solvent. This solvent is withdrawn at the top of the column with a purity of 20 to 99 wt % and contains all low-boiling components. The requirements concerning the purity of the TDI product stream is described analogously to that in EP 1 371 635 B1. EP 1 575 907 B1 describes a process for purifying TDI in combination with a further concentration of the higher boiling components. The TDI fraction generated in the concentration of the higher boiling components is sent back to the dividing wall column for recovery. As a consequence thereof this dividing wall column has 2, optionally 3 feeds and depending on the arrangement a dividing wall which runs right into the column bottom.
Since the obtained TDI is generally the commercial product high demands for maintaining quality are placed on it. Any malfunction of the dividing wall column results in off-specification product. The other stream obtained must also achieve the required specifications in terms of their composition since they are often recycled into the production process for reuse. If the specifications are not achieved this can result in inefficiently high circuit streams or in yield losses, in particular when for example in the case of the production of isocyanates and in particular TDI the recovered solvent does not achieve the required purity for use in the reaction stage. This places high demands on the dimensioning of the column and on the fabrication thereof.
The dimensioning of dividing wall columns is generally effected through stationary simulators, such as are commercially available for example from AspenTech. The simulation may generally be effected by interconnection of conventional columns (cf. Chr. Hiller “Auslegung von Trennwandkolonnen: Modellierungsansätze and experimentelle Validierung”, Chapter 2 of the lecture notes of the dividing wall column symposium of Oct. 13, 2011 at the Institute for Process and Plant Technology of the Technical University of Hamburg-Harburg). FIG. 5 shows such an arrangement. Column 1 (K-1) represents the portion above the dividing wall. The region of the dividing wall is depicted by two further, separate columns (K-2, K-3). K-4 mirrors the region below the dividing wall. The apparatuses belonging to the column such as condensers and evaporators are advantageously described as separate models but may also be integrated into the column models K-1 and K-4 depending on the simulation program used. Through connecting material streams the individual columns K-1 to K-4, the condenser and the evaporator become one dividing wall column. As column models equilibrium step models or else non-equilibrium step models may be employed. Equilibrium state models are the simpler and more flexible approach here since they require fewer material data. However, these models reach their limits for strongly non-ideal material systems.
Essential for the fitness for purpose of a dividing wall column is the division of the vapors from the lower common portion K-4 between the two sides of the dividing wall K-2 and K-3. This division is only dependent on the ratio of the pressure drops left and right of the dividing wall. In real columns the pressure drop depends on the chosen column internals and on gas and liquid load. For simulation as a basis for dimensioning it is thus necessary to use process models able to calculate pressure drop as a function of the hydrodynamics of the column internals used later. Only then is an assured forecast of the separation performance of the column, especially for partial load behavior and altered operating parameters such as alteration of the composition of the feed, possible.
The validity of the simulation should be verified by experimental data, particularly when the description of the material data cannot be achieved with the required precision or else product properties not applicable to a simulation (for example color) must be achieved. Studies (cf. G. Niggemann et al., Ind. Eng. Res. 2010, 49, 6566-6577) revealed good agreement between laboratory and simulation results, in particular with respect to the gas distribution below the dividing wall. The test setup consists of a distillation column having an internal diameter of 68 mm and containing a plurality of Montz B1-500 mass transfer elements. In addition to the good agreement of experimental and calculated results this publication reports on the influence of heat transport on the separator behavior of the column. However, this influence is limited to laboratory columns since relative to throughput the surface area of the dividing wall is much greater here than for industrial columns Another paper (Chr. Hiller et al., Heat Mass Transfer (2010) 46, 1209-1220) also confirms good agreement of experimental data with the simulation results using non-equilibrium models.
For industrial implementation of dividing wall columns various manufacturers (for example Sulzer Chemtech, Montz) market the relevant column internals such as liquid collectors and redistributors. The dividing wall may either be welded to the outer column jacket or else loosely inserted, as described in EP 1 008 577 A1.
In conclusion it should be noted that the configuration of dividing wall columns for large industrial scale applications is typically based on simulations whose agreement with experimental data is generally regarded as sufficient. However, in operational practice it has been found that, surprisingly, this is not always the case. It was thus observed that for instance the purity of TDI withdrawn as a sidestream in the region of the dividing wall can in reality occasionally deviate significantly from the purity to be expected from simulation results. The same applies to the purity of the solvent withdrawn as a low-boiling component at the top of the dividing wall column. There was therefore a need for further improvements to existing dividing wall columns.